Semiconductor modules have various applications, such as the LSI, IGBT, power semiconductor, radio-wave/optical communication semiconductor, laser, LED and sensor. Their structures significantly vary depending on the required performances for these applications. A semiconductor module is an extremely sophisticated precision device composed of a plurality of members made of various materials having different coefficients of linear expansion along with different degrees of thermal conductivity. The heat dissipation substrate used in the package of the semiconductor module also has a wide variety of composite materials and shapes proposed thus far.
The heat dissipation substrate for semiconductor modules must have a suitable coefficient of linear expansion to secure the performance and life of semiconductor devices in the process of manufacturing the package and soldering the semiconductor devices. It also needs to have a high degree of thermal conductivity in order to dissipate heat from the semiconductor devices and cool them to secure their performance and life. It is also extremely important that the substrate should allow a satisfactory plating for the bonding of various members and semiconductor devices.
Heat dissipation substrates can be roughly classified by their forms as follows: a sub-mount of a few millimeters square with a thickness of 1 mm or less; a flat plate; a threaded flat plate; and a three-dimensional shape. A manufacturing method by which those shapes can be easily obtained is desired.
Originally, copper (Cu) was used for heat dissipation substrates. However, due to the recent improvement in the performance of the semiconductor modules, the amount of heat generation has increased so much that the use of Cu has caused problems in relation to the manufacturing process and durability of the package as well as the operation life of the semiconductor devices, since the coefficient of linear expansion of Cu is too large. Thus, there has been an increasing demand for a heat dissipation substrate having a coefficient of linear expansion that matches with high-performance semiconductor modules.
To address this problem, CuW and CuMo have been developed (Patent Literature 1), whose coefficient of linear expansion can be modified or adjusted so as to match with the coefficient of linear expansion for high-performance semiconductor modules. AISiC has also been developed (Patent Literature 2) for applications which require lightweight materials. However, all of these composite materials have the problem that their thermal conductivity does not exceed 320 W/m·K and is lower than that of Cu when they have suitable coefficients of linear expansion for semiconductor modules.
Accordingly, various composite materials for heat dissipation substrates have been researched and developed, with the aim of creating a material whose coefficient of linear expansion is within the range covered by CuW, CuMo and AISiC (6.5 ppm/K or higher and 15 ppm/K or lower) and one whose thermal conductivity is equal to that of Cu (393 W/m·K) or even higher and exceeds that of Ag (420 W/m·K) which has the highest thermal conductivity among metallic elements.
In addition to the coefficient of linear expansion and thermal conductivity, there is another important property for heat dissipation substrates: the quality of plating. When a manufacturer of semiconductor modules solders a semiconductor device or insulating sheet, if there are many voids at the bonded interface, those voids will block the flow of heat, causing separation of or damage to the semiconductor device or insulating sheet. Therefore, heat dissipation substrates need to have a surface layer which has few defects and thereby allows for the Ni-based final plating which enables a satisfactory soldering.
The Ni-based final plating is performed in various forms for securing its quality. To deal with those forms, the plating in some cases is performed by the manufacturer of the heat dissipation substrate or in the other cases by the package manufacturer. A variety of Ni-based plating methods, soldering materials, soldering conditions, etc. have been developed to meet the quality requirements. In order to secure the quality of the Ni-based final plating in these developing activities, it is extremely important that the heat dissipation substrate should have few defects in its surface layer. To achieve this, various types of heat dissipation substrates have been developed.
Since the Ni-based final plating can be performed in various forms, the values of the coefficient of linear expansion and thermal conductivity measured before the Ni-based final plating are generally used as the reference properties of a heat dissipation substrate made of a composite material.
Heat dissipation substrates made of Cu have few defects in their surface layer, so it is easy to have a satisfactory Ni-based final plating formed on it. However, in the case of machine-worked or polished products made of CuW or CuMo, the problem of the defects in the surface layer easily occurs if the relative density of the product is low; it is commonly said that, for practical purposes, the relative density should be equal to or higher than 99% of the true density. By contrast, in the case of clad products (metal-coated products), the problem related to the final Ni-plating is avoided, since the surface layers formed on the upper and lower surfaces of the product are Cu layers.
As for AISiC, there is the problem that the plating cannot be easily formed on ceramic sites (SiC) even if the relative density is equal to or higher than 99% of the true density. However, even when the composite material has pinholes (micro-sized holes present on its surface) or similar defects or has SiC sites which obstruct the plating, the Ni-based final plating can be satisfactorily performed if pure aluminum foil or a layer of aluminum made by infiltration is provided on the surface of the composite material in the process of creating this material.
In recent years, due to the rapid progress and performance improvement of semiconductor modules, the amount of heat generated from the semiconductor devices is increasing, with the corresponding increase in the importance of the heat-control measure. Thus, there has been a strong demand for a novel, high-quality heat dissipation substrate which has: a coefficient of linear expansion which can match with that of the semiconductor modules; a high degree of thermal conductivity; and a surface condition which enables a satisfactory soldering that can pass the void assessment test at the bonded interface which is stricter than the solder wettability test.
Heat dissipation substrates made of metal-diamond composites have the possibility of achieving a high degree of thermal conductivity and are promising as a heat dissipation substrate for high-performance semiconductor modules. Therefore, various efforts for research and development have been made and reported thus far on this subject.
In the case of using only metal and diamond, the wettability of the metal to the diamond is so poor that it is difficult to produce a composite material for heat dissipation substrates by the liquid metal infiltration method or sintering method which are conventionally employed for the production of CuW or CuMo. Meanwhile, it has been reported that a high degree of thermal conductivity can be achieved by an ultrahigh pressure sintering method (Patent Literature 3) in which a powder of Cu and diamond is canned and sintered at high temperatures under a high pressure of 50000 atm. By this method, a composite material having a high relative density can be obtained. However, due to its diamond-rich composition, its coefficient of linear expansion is too low, and its production cost is high. Additionally, the process of slicing and grinding a block material is necessary to shape the material into the form of a product. Such a process creates defects, which cause a problem related to the quality of the Ni-based final plating and consequently limit the field of application.
It has been reported that a product obtained by sintering a green compact made of a powder mixture of a principal metal, additional metal and diamond has a high degree of thermal conductivity due to the carbide of the additional metal formed on the surface of the diamond (Patent Literature 4). However, an alloy composite obtained by this sintering method is unstable and cannot have a high real density. Consequently, a large number of pinholes are formed on the surface of the alloy composite, making it impossible to secure a satisfactory quality of the Ni-based final plating. Therefore, no alloy composite which can be used as the heat dissipation substrate has yet been obtained.
It has been reported that a high degree of thermal conductivity can be achieved by a manufacturing method in which a metal is infiltrated in a skeleton composed of diamond powder with the film of the carbide of additional metal formed on its surface layer (Patent Literature 5). This method can achieve a higher real density and higher thermal conductivity than the sintering method. However, the obtained products vary in composition due to the unstable structure of the skeleton. Additionally, the infiltrated metal remaining at the periphery needs to be removed using a diamond grinding wheel. This grinding process causes the chipping or grain separation of diamond from the surface of the composite; in particular, the interface separation between the diamond and metal occurs. Accordingly, it is impossible to perform the Ni-based final plating with the necessary level of quality for heat dissipation substrates even if a metal is deposited. Therefore, the obtained composite cannot be used as a heat dissipation substrate.
It has been reported that a high degree of thermal conductivity can be obtained by sintering a green compact of Cu-plated diamond powder by a spark plasma sintering (SPS) process (Patent Literature 6). However, the Cu-plating of the diamond powder is very expensive. Additionally, to achieve a high degree of thermal conductivity by the SPS electrical sintering method, the sintering process must be continued for a considerable period of time, which lowers productivity. Another problem is that the diamond is occasionally exposed on the surface layer, making it impossible to secure the necessary level of quality of the Ni-based final plating for a satisfactory soldering.
It has also been reported that a product obtained by pressure infiltration of Al—Si—Mg alloy into a skeleton composed of diamond powder coated with SiC ceramic (Patent Literature 7) has a high degree of thermal conductivity and yet can satisfy the quality requirement of the Ni-based final plating due to the effect of the film of the infiltrated metal formed on its surface layer. However, in the case of a thin heat dissipation substrate, the product is unsuitable as the heat dissipation substrate since a layer made of the infiltrated metal, which is a poor conductor of heat, is present on its surface layer. This method is also uneconomical since the process of forming a layer of the infiltrated metal on the surface layer using a precision jig is extremely difficult and prevents inexpensive production of the composite. Additionally, the film of the infiltrated metal is not always suitable for the Ni-based final plating. Furthermore, this method is only applicable to aluminum alloy, and the content of the aluminum alloy must be 60% or less to secure the required level of stability of the skeleton. Therefore, this technique is only available for limited forms of heat dissipation substrates used in limited applications.
There has also been a report on a package produced by silver-brazing a pure copper plate onto a composite created by infiltrating Cu into a green compact of diamond powder coated with metal or ceramic (Patent Literature 8). However, the process of coating the diamond powder with metal or ceramic is expensive. Additionally, this method is uneconomical since it has many production steps, including the removal of the infiltrated metal remaining at the periphery using a diamond grinding wheel, followed by the silver brazing of the copper plate. When the pure copper plate is silver-brazed onto the heat dissipation substrate made of the metal-diamond composite, the copper reacts with the silver-brazing material and turns into an alloy, forming a low-thermal-conductivity layer. Additionally, even when a thick copper plate is used, voids or other defects occur in the brazing portion. Due to these problems, no composite which can be used as a heat dissipation substrate has yet been produced on a commercial basis.